2ndDISCUSSION FORUM ON INDUSTRIAL ECOLOGY AND LIFE-CYCLE MANAGEMENT
Coimbra, March 5-6 2015
ENERGY
REQUIREMENTS
FOR
THE
CONTINUOUS
BIOHYDROGEN PRODUCTION FROM SPIROGYRA BIOMASS IN A
SEQUENTIAL BATCH REACTOR
Joana Ortigueira 1* , Ana Ferreira 2 , Carla Silva 2, Luísa Gouveia1 and Patrícia Moura1
1: Unidade de Bioenergia, Laboratório Nacional de Energia e Geologia, Estrada do Paço do
Lumiar, 22; 1649-038 Lisboa, Portugal
* e-mail: [email protected]
2: IDMEC/LAETA, Instituto Superior Técnico, Universidade de Lisboa, Av. RoviscoPais, 1;
1049-001 Lisboa, Portugal
Keywords: Biohydrogen, Microalgae, Life-Cycle Analysis.
Abstract
The current energy market requires urgent revision for the introduction of renewable,
less-polluting and inexpensive energy sources. Biohydrogen (bioH 2 ) is considered to be
one of the most appropriate options for this model shift, being easily produced through
the anaerobic fermentation of carbohydrate-containing biomass. Ideally, the feedstock
should be low-cost, widely available and convertible into a product of interest.
Microalgae are considered to possess the referred properties, being also highly valued for
their capability to assimilate CO2 [1]. The microalga Spirogyra sp. is able to accumulate
high concentrations of intracellular starch, a preferential carbon source for some bioH 2
producing bacteria such as Clostridium butyricum [2]. In the present work, Spirogyra
biomass was submitted to acid hydrolysis to degrade polymeric components and increase
the biomass fermentability. Initial tests of bioH2 production in 120 mL reactors with C.
butyricum yielded a maximum volumetric productivity of 141 mL H2/L.h and a H2
production yield of 3.78 mol H2/mol consumed sugars. Subsequently, a sequential batch
reactor (SBR) was used for the continuous H 2 production from Spirogyra hydrolysate.
After 3 consecutive batches, the fermentation achieved a maximum volumetric
productivity of 324 mL H2/L.h, higher than most results obtained in similar production
systems [3] and a potential H2 production yield of 10.4 L H 2/L hydrolysate per day. The
H2 yield achieved in the SBR was 2.59 mol H2/mol, a value that is comparable to those
attained with several thermophilic microorganisms [3], [4].
In the present work, a detailed energy consumption of the microalgae value-chain is
presented and compared with previous results from the literature. The specific energy
requirements were determined and the functional unit considered was gH2 and MJH2. It was
possible to identify the process stages responsible for the highest energy consumption
during bioH2 production from Spirogyra biomass for further optimisation.
J. Ortigueira, A. Ferreira, C. Silva, L. Gouveia and P. Moura
1. INTRODUCTION
Biofuels are regarded as a viable alternative to fossil fuels for the production of renewable
energy. Special attention has been given to biomass-derived fuels thanks to their
renewable and largely non-polluting qualities. Biohydrogen (bioH2 ) is one of such fuels,
being easily convertible into energy through combustion, a process which yields solely
water as sub-product [5]. BioH2 production can be attained by anaerobic fermentation of
carbohydrate-containing biomass and originates a highly rich biogas containing both H 2
and CO2 [6]. A prime example of a feedstock adequate for bioH2 production is microalgal
biomass. Microalgae are photosynthetic organisms able to assimilate atmospheric CO 2 and
store both lipids and carbohydrates in their intracellular space. They are also highly
productive allowing for a near daily harvest and, unlike higher plant cultures, require no
arable land or potable water [7]. BioH2 production has already been successfully achieved
by the authors, using Scenedesmus obliquus [8], [9], Chlorella vulgaris [10] and
Spirogyra sp. biomass [2]. Spirogyra, in particular, is able to accumulate starch, a
preferential substrate for anaerobic fermentation by certain bacterial strains, at very high
concentrations [2]. In this work, the production of bioH2 from Spirogyra biomass by
Clostridium butyricum was evaluated in small-scale batch reactors and a bench-scale
sequential batch reactor. Both processes were compared in terms of their H2 yield,
production rate and overall energy consumption.
2. MATERIALS AND METHODS
The Spirogyra biomass used in this work had the following average composition (% (w/w)
dry weight basis): 45.1% total sugars, 22% crude protein, 3.6% fat, 25.9% ash and 3.4%
others (by difference). The microalga was cultured and harvested as already described [2].
Biomass hydrolysis was performed with H2SO4 1N (60 min, 121 ºC). Small scale
fermentation was undertaken in 120 mL serum flasks containing 20 mL of MCM medium
[11]. Bench-scale sequential batch fermentation was performed in a lab scale double jacketed
reactor (1.65 L) with a total medium volume of 500 mL (10 g/L of total sugars, 37 ºC, 150
rpm). After the first batch assay, 250 mL of the medium were replaced with a 1:1 mixture of
hydrolysate and concentrated MCM. The produced biogas was collected and stored in
inverted serum flasks filled with water, and quantified by displacement of the liquid phase.
Biomass dry weight was determined throughout the fermentation. Gas samples were analysed
by GC and the fermentate samples by HPLC [9]. The final energy consumption inventory
associated with the microalga culturing, harvesting, drying, hydrolysis and fermentation was
assessed based on direct equipment energy measurements. The results are expressed in
MJ/MJH2.
3. RESULTS AND DISCUSSION
H2 production from Spirogyra sp. hydrolysate was first attempted in a small set-up consisting
of individual flasks with the purpose of evaluating whether C. butyricum was able to
successfully convert the sugars made available by the acid hydrolysis. The fermentation
results are displayed in figure 1.
2
J. Ortigueira, A. Ferreira, C. Silva, L. Gouveia and P. Moura
A
B
Figure 1. Time-course of H2 production, sugar consumption and cell dry weight in: A) small-scale batch
reactor; B) bench-scale sequential batch reactor (□ – H2 ; ♦ – total sugars; ∆ – cells).
As seen in figure 1 (A), H2 production occurred rapidly between 0 and 12 hours of incubation
with no visible lag phase. The maximum H2 production was achieved at 42 hours of
fermentation (2.1 L H2/L). The maximum H2 percentage in the biogas produced was 28%
(v/v). The highest H2 production rate (141 mL/L.h) was detected from up to 12 hours and
corresponded to a H2 yield of 3.78 mol H2/ mol glucose equivalents. These results show that
not only H2 production from Spirogyra hydrolysate was viable as it was comparable or higher
to already published results [8], [9].
With the objective of scaling-up the bioH2 production, a sequential batch reactor (SBR) was
set-up. The use of a sequential batch system maintains the concentration of biomass inside the
reactor in a quasi-exponential status, virtually eliminating the lag-phase between consecutive
batches and lessening the operation time of each batch. This enables to increase the number of
batches per day and the overall H2 production rate. Figure 1 (B) depicts the results of bioH2
production from Spirogyra hydrolysate in SBR during three consecutive batches. In
comparison to the small-scale batch reactor, the H2production rate increased almost two-fold
(324 mL/L.h) and the biogas produced was richer in H2 (>50% (v/v)). The bioH2 production
did not change significantly during the consecutive batches, displaying a steady, uninterrupted
production profile up to 4.4 L H2/L.
Table 1 summarises the inventory of both production processes and the energy consumption
associated to each stage of H2 production. The results are presented in MJ per g of H2
produced.
3
J. Ortigueira, A. Ferreira, C. Silva, L. Gouveia and P. Moura
Table 1. Inventory results of bioH2 production from Spirogyra biomass (MJ/gH2)
Production stage
BioH2 production in
small-scale batch reactor
BioH2 production
in SBR
0.11
1.87
1.71
1.96
0.03
0.55
0.07
3.09
Microalga culture
Biomass harvesting and drying
Biomass hydrolysis
Fermentation
In order to show the results in MJ per MJ of hydrogen produced, a lower heating value of 120
MJ/kg was used [1], [2]. A total energy consumption of 47 and 31 MJ/MJH2 was obtained in
the small-scale batch reactor and the SBR, respectively. Previous studies on the fermentation
of dried and ground microalgal biomass achieved total energy consumption values of 88
MJ/MJH2 with Scenedesmus obliquus as feedstock and 207 MJ/MJ H2 with Spirogyra sp.,
values which are visible higher than those attained in this study [1], [2]. The use of less
energy consuming harvesting and drying procedures (electrocoagulation and solar
dewatering) and the use of a simpler culture medium contributed for this energy consumption
decrease. In the small-scale process, the production and processing of the microalgal biomass
(harvesting, drying, hydrolysis) was responsible for a considerable energy consumption (30
MJ/MJH2), in accordance to what was already reported by other authors [12]. In contrast, the
fermentation was clearly the stage which consumed more energy in the SBR. This result is
directly related to the high energy consumption of the heating bath used for controlling the
reactor temperature. Together, the biomass hydrolysis and the fermentation stages accounted
for 85% (26 MJ/MJH2) of the total energy consumption in the SBR.
The comparison between small-scale and bench-scale allowed us to assess that the SBR
improved the cumulative H2 production, the H2 production rate and the global energy
consumption. Although it is still necessary to reduce the ratio of energy input per energy
output, the increase in the scale of bioH2 production allowed for a reduction of 34% of the
energy requirements. The comparison of the results obtained in this work with others
already published [2] shows a clear improvement in the process performance, likely due to
the refinement of the microalga harvesting process, the culture medium optimisation and
the bioconversion efficiency.
4. CONCLUSIONS
The purpose of the current study was to evaluate the effect of scaling-up the bioH2
production from microalgal biomass in the energy requirements and production yield of
the process. The fermentation results show that Spirogyra biomass is an adequate
feedstock for the fermentation by C. butyricum, achieving H2 production yields close to
the maximum theoretical value. The SBR system improved significantly the H2 production
yield (from 2.1 to 4.4 L H 2 /L) and H2 production rate (from 141 to 324 mL/L.h), while
supporting at the same time the operation in a continuous mode. The energy inventory
analysis revealed that the process scale-up decreased the energy consumption in 34%. It is
4
J. Ortigueira, A. Ferreira, C. Silva, L. Gouveia and P. Moura
possible that pursuing with the optimisation of the fermentation stage, the energy
requirements may decrease to values which make the biological H2 production more
sustainable.
REFERENCES
[1]
A. F. Ferreira, J. Ortigueira, L. Alves, L. Gouveia, P. Moura, and C. Silva,
“Biohydrogen production from microalgal biomass: Energy requirement, CO2
emissions and scale-up scenarios,” Bioresour. Technol., vol. 144, pp. 156–164, 2013.
[2] R. Pacheco, a. F. Ferreira, T. Pinto, B. P. Nobre, D. Loureiro, P. Moura, L. Gouveia,
and C. M. Silva, “The production of pigments & hydrogen through a Spirogyra sp.
biorefinery,” Energy Convers. Manag., vol. 89, pp. 789–797, 2015.
[3] T.-A. Nguyen, K.-R. Kim, M.-T. Nguyen, M. S. Kim, D. Kim, and S. J. Sim,
“Enhancement of fermentative hydrogen production from green algal biomass of
Thermotoga neapolitana by various pretreatment methods,” Int. J. Hydrogen Energy,
vol. 35, pp. 13035–13040, 2010.
[4] L. Dipasquale, C. Gallo, F. Monica, A. Gambacorta, G. Picariello, and A. Fontana,
“Hydrogen production by the thermophilic eubacterium Thermotoga neapolitana from
storage polysaccharides of the CO2-fixing diatom Thalassiosira weissflogii,” Int. J.
Hydrogen Energy, vol. 37, no. 17, pp. 12250–12257, 2012.
[5] F. Hawkes, “Sustainable fermentative hydrogen production: challenges for process
optimisation,” Int. J. Hydrogen Energy, vol. 27, no. 11–12, pp. 1339–1347, Nov. 2002.
[6] G. Davila-vazquez, S. Arriaga, F. Alatriste-Mondragon, A. León-Rodriguez, L.
Rosales-colunga, and E. Razo-Flores, “Fermentative biohydrogen production: trends
and perspectives,” Rev. Environ. Sci. Biotechnol., vol. 7, pp. 27–45, 2008.
[7] L. Gouveia and A. C. Oliveira, “Microalgae as a raw material for biofuels production,”
J. Ind. Microbiol. Biotechnol., vol. 36, no. 2, pp. 269–74, 2009.
[8] A. P. Batista, P. Moura, P. A. S. S. Marques, J. Ortigueira, L. Alves, and L. Gouveia,
“Scenedesmus obliquus as feedstock for biohydrogen production by Enterobacter
aerogenes and Clostridium butyricum,” Fuel, vol. 117, pp. 537–543, 2014.
[9] J. Ortigueira, L. Alves, L. Gouveia, and P. Moura, “Third generation biohydrogen
production by Clostridium butyricum and adapted mixed cultures from Scenedesmus
obliquus microalga biomass,” Fuel. Pending approval of minor revisions, 2015.
[10] C. Liu, C. Chang, C. Cheng, and D. Lee, “Fermentative hydrogen production by
Clostridium butyricum CGS5 using carbohydrate-rich microalgal biomass as
feedstock,” Int. J. Hydrogen Energy, vol. 37, no. 20, pp. 15458–15464, 2012.
[11] J. Ortigueira, T. Pinto, L. Gouveia, and P. Moura, “Production and Storage of
Biohydrogen in Sequential Batch Fermentation from Spyrogira by Clostridium
butyricum,” Energy. Under revision, 2015.
[12] A. F. Ferreira, J. Ortigueira, L. Alves, L. Gouveia, P. Moura, and C. M. Silva, “Energy
requirement and CO2 emissions of bioH2 production from microalgal biomass,”
Biomass and Bioenergy, vol. 49, no. 1988, pp. 249–259, 2012.
5